D-psicose is a natural sugar present in a trace amount in fruits such as figs and raisins or isomerization products of molasses or glucose. D-psicose is a monosaccharide with a sweetness of 70% relative to sugar. D-psicose was reported to be a sweetener that contains few or no calories because it is not metabolized and that has little effect on body weight gain because it functions to inhibit the formation of body fat. D-psicose was also reported to inhibit the activity of enzymes involved in lipid synthesis, resulting in a reduction in abdominal obesity (Matuo, T. et al., Asia Pac. J. Clin. Nutr., 10, 223-237, 2001; Matsuo. T. and K. Izumori. AsiaPac. J. Clin. Nutr., 13, 5127, 2004). According to a recently published report, D-psicose has no influence on hyperglycemia and assists in dental health due to its non-cariogenic and anti-cariogenic properties.
Thus, D-psicose has attracted increasing attention as a sweetener due to advantages thereof. However, since D-psicose is a rare monosaccharide that exists only in a trace amount in nature, there is a need to develop an efficient technique for D-psicose production that can be applied to the food industry.
Conventional methods for D-psicose production are divided into chemical methods and biological methods. According to the chemical methods, psicose is produced from fructose based on the catalysis of molybdate ions. The chemical methods are disadvantageous in that D-psicose is produced in a very small amount during treatment of molasses or isomerization of glucose, considerable costs are incurred, and large amounts of by-products are generated. According to the biological methods, D-psicose is produced from D-fructose using a D-psicose 3-epimerase. Problems of the biological methods are very low yields and high production costs.
In an effort to solve such problems, the present inventors reported a method for economically producing D-psicose comprising isomerizing glucose to fructose and reacting the fructose with immobilized bacterial cells producing a D-psicose 3-epimerase (Korean Patent Application No. 10-2009-0118465), prior to the filing of the present application. The present inventors also reported a method for economically producing pure D-psicose in a crystalline state without using an organic solvent such as ethanol. D-psicose produced by this method is flowable during production and has commercial value (Korean Patent Application No. 10-2010-0027546).
However, the methods for producing D-psicose using a D-psicose 3-epimerase (also referred to simply as “psicose epimerase”) (hereinafter, the methods are referred to simply as “enzymatic conversion methods”) are still limited in yield. Thus, an effort needs to be made to increase the yield of D-psicose for industrial mass production and improve the conversion rate of D-fructose to D-psicose by a psicose epimerase to lower production cost.
Under such circumstances, the present inventors have earnestly conducted research to develop a method for improving the conversion rate of D-fructose to D-psicose using a psicose epimerase, and as a result, found that use of an aluminate or iodate in an enzymatic conversion method can significantly improve the conversion rate. The present invention has been accomplished based on this finding.